Catalyst for selective conversion of oxygenates to aromatics

ABSTRACT

A catalyst composition comprises a self-bound zeolite and a Group 12 transition metal selected from the group consisting of Zn, Cd, or a combination thereof, the zeolite having a silicon to aluminum ratio of at least about 10, the catalyst composition having a micropore surface area of at least about 340 m 2 /g, a molar ratio of Group 12 transition metal to aluminum of about 0.1 to about 1.3, and at least one of: (a) a mesoporosity of greater than about 20 m 2 /g; (b) a diffusivity for 2,2-dimethylbutane of greater than about 1×10 −2  sec −1  when measured at a temperature of about 120° C. and a 2,2-dimethylbutane pressure of about 60 torr (about 8 kPa).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of provisional U.S. Application Nos. 61/918,984, 61/918,994, and 61/919,013, each filed on Dec. 20, 2013; each of which are incorporated by reference herein in their entirety.

FIELD

This invention relates to catalysts for converting oxygenates to aromatics and methods for using such catalysts.

BACKGROUND

A variety of industrial processes are known for conversion of low boiling carbon-containing compounds to higher value products. For example, methanol to gasoline (MTG) is a commercial process that produces gasoline from methanol using ZSM-5 catalysts. In the MTG process, methanol is first dehydrated to dimethyl ether. The methanol and/or dimethyl ether then react in a series of reactions that result in formation of aromatic, paraffinic, and olefinic compounds. The resulting product consists of liquefied petroleum gas (LPG) and a high-quality gasoline comprised of aromatics, paraffins, and olefins. The typical MTG hydrocarbon product consists of about 40-50% aromatics plus olefins and about 50-60% paraffins.

U.S. Pat. Nos. 6,423,879 and 6,504,072 disclose processes for the selective production of para-xylene which comprises reacting toluene with methanol in the presence of a catalyst comprising a porous crystalline material having a diffusivity for 2,2-dimethylbutane of less than about 10⁻⁴ sec⁻¹ when measured at a temperature of about 120° C. and a 2,2-dimethylbutane pressure of about 60 torr (about 8 kPa). The porous crystalline material is preferably a medium-pore zeolite, particularly ZSM-5, which has been severely steamed at a temperature of at least about 950° C. and which has been combined with about 0.05 to about 20 wt % of at least one oxide modifier, preferably an oxide of phosphorus, to control reduction of the micropore volume of the material during the steaming step. The porous crystalline material is normally combined with a binder or matrix material, preferably silica or a kaolin clay.

U.S. Pat. No. 4,088,706 describes a method for converting methanol to para-xylene. The method includes exposing a feed to a zeolite catalyst that is modified to include boron and/or magnesium.

U.S. Pat. No. 4,584,423 describes a method for xylene isomerization using a preferably alumina-bound zeolite catalyst containing a Group 2 or Group 12 metal. A feed containing a mixture of aromatic compounds including ethylbenzene is exposed to the catalyst for conversion of ethylbenzene to other compounds while reducing or minimizing the amount of xylene conversion.

U.S. Pat. No. 3,894,104 describes a method for converting oxygenates to aromatics using zeolite catalysts impregnated with a transition metal. Yields of aromatics relative to the total hydrocarbon product are reported to be as high as about 58% with a corresponding total C5+ yield as high as about 73%.

U.S. Patent Application Publication No. 2013/0281753 describes a phosphorous modified zeolite catalyst. The phosphorous modification reduces the change in alpha value for the catalyst after the catalyst is exposed to an environment containing steam. The phosphorous modified catalysts are described as being suitable, for example, for conversion of methanol to gasoline boiling range compounds.

SUMMARY

In one aspect, a catalyst composition is provided to include a self-bound zeolite and a Group 12 transition metal selected from the group consisting of Zn, Cd, or a combination thereof, the zeolite having a silicon to aluminum ratio of at least about 10, the catalyst composition having a micropore surface area of at least about 340 m²/g, a molar ratio of Group 12 transition metal to aluminum of about 0.1 to about 1.3, and at least one of: (a) a mesoporosity of greater than about 20 m²/g; (b) a diffusivity for 2,2-dimethylbutane of greater than about 1×10⁻² sec⁻¹ when measured at a temperature of about 120° C. and a 2,2-dimethylbutane pressure of about 60 torr (about 8 kPa).

In another aspect, a catalyst composition is provided to include a self-bound zeolite and a Group 12 transition metal selected from the group consisting of Zn, Cd, or a combination thereof, the zeolite having a silicon to aluminum ratio of at least about 20, and the catalyst composition having a micropore surface area of at least about 340 m²/g, a molar ratio of Group 12 transition metal to aluminum of about 0.1 to about 1.3, a mesoporosity of greater than about 20 m²/g, and a diffusivity for 2,2-dimethylbutane of greater than 1×10⁻² sec⁻¹ when measured at a temperature of about 120° C. and a 2,2-dimethylbutane pressure of about 60 torr (about 8 kPa).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Aromatics and olefins are valuable chemical products. Although processes for converting methanol to gasoline are known, such processes may not enhance or maximize the production of valuable aromatics and/or olefins. Thus, a catalyst or process that can further increases the amount of aromatic and/or olefinic products generated from conversion of methanol or other oxygenates, while minimizing paraffin formation, would be commercially attractive.

In various aspects, catalysts described herein can be used to convert an oxygenate feed into aromatics and/or olefins with improved yield of one or more desired components relative to the total hydrocarbon product generated in the conversion reaction. The total hydrocarbon product from an oxygenate conversion reaction is defined as the yield of hydrocarbon or hydrocarbonaceous products. Thus, the yield of compounds such as water, coke, or other non-hydrocarbonaceous products is excluded from the total hydrocarbon yield. The improved yield can be identified as an improved yield of aromatics relative to the total hydrocarbon product; an improved combined yield of aromatics and olefins relative to the total hydrocarbon product; an improved yield of aromatics relative to the yield of C5+ (liquid) product in the total hydrocarbon product; or a combination thereof.

One example of the difficulties in using conventional catalysts for converting oxygenates, such as methanol, to gasoline (MTG) is the formation of substantial amounts of paraffins in the liquid hydrocarbon product. C5+ paraffins, such as C5-C8 paraffins, are an acceptable component in a conventional naphtha or gasoline product. However, although such paraffins are acceptable, C5-C8 paraffins are otherwise a relatively low value product. Generation of lower value products from a catalyzed synthesis process reduces the overall value of the process.

In contrast to conventional methods, conversion of oxygenates using catalysts as described herein can enhance the relative amount of aromatics and olefins generated during conversion. In other words, the amount of paraffins generated in the total hydrocarbon product can be reduced, and/or the amount of paraffins in the liquid portion (C5+) of the hydrocarbon product can be reduced.

The enhanced yield of desirable products can be identified in several ways. One way of identifying the enhanced yield of desirable products is to consider either the amount of aromatics produced relative to the total hydrocarbon product, or to consider the aromatics plus olefins produced in the total hydrocarbon product. Increasing the amount of aromatics generated can indicate production of higher value components based on the value of various aromatic compounds for applications other than as a fuel. Typically aromatics are produced as a mixture of aromatics having various numbers of carbon atoms. Performing a separation on the mixture of aromatics can allow for recovery of the higher value aromatics in the mixture.

Increasing the combined amount of aromatics and olefins can also indicate an increase in the value of the products generated from a reaction. At least part of the olefins generated in the total hydrocarbon product can correspond to C2-C4 olefins. These olefins can be suitable for use as raw materials for a variety of polymer synthesis reactions. Thus, even though the increase in chain length for C2-C4 olefins is small relative to an initial methanol feed, such C2-C4 olefins can still represent a higher value product than paraffins generated by conversion of methanol (or another oxygenate).

As an alternative to using the combined amount of aromatics plus olefins, the amount of aromatics generated relative to the liquid yield of the total hydrocarbon product can also indicate production of a higher value mixture of products. The liquid portion or yield for the hydrocarbon products typically refers to the portion of the hydrocarbon products that contain at least 5 carbons (C5+ compounds). The difference between the weight percent of aromatics in the total hydrocarbon product versus the weight percent of liquid product in the total hydrocarbon product usually corresponds to paraffinic compounds. Thus, reducing and/or minimizing the amount of difference between the liquid product yield and the aromatic product yield can correspond to production of a higher value mixture of hydrocarbon products.

Catalyst for Oxygenate to Aromatics Conversion

In various aspects, a transition metal-enhanced zeolite catalyst composition is provided, along with methods for use of the transition metal enhanced catalyst for conversion of oxygenate feeds to aromatics and olefins with enhanced overall yield and/or enhanced aromatics yield. In some cases, the present catalyst composition is alternatively referred to as being self-bound. The terms “unbound” and “self-bound” are intended to be synonymous and mean that the present catalyst composition is free of any of the inorganic oxide binders, such as alumina or silica, frequently combined with zeolite catalysts to enhance their physical properties.

The zeolite employed in the present catalyst composition generally comprises at least one medium pore aluminosilicate zeolite having a Constraint Index of 1-12 (as defined in U.S. Pat. No. 4,016,218). Suitable zeolites include zeolites having an MFI or MEL framework, such as ZSM-5 or ZSM-11. ZSM-5 is described in detail in U.S. Pat. Nos. 3,702,886 and RE 29,948. ZSM-11 is described in detail in U.S. Pat. No. 3,709,979. Preferably, the zeolite can be ZSM-5.

Generally, a zeolite having the desired activity can have a silicon to aluminum molar ratio of about 10 to about 300, such as about 15 to about 100 or about 20 to about 40. For example, the silicon to aluminum ratio can be at least about 10, such as at least about 20, or at least about 30, or at least about 40, or at least about 50, or at least about 60. Additionally or alternately, the silicon to aluminum ratio can be about 300 or less, such as about 200 or less, or about 100 or less, or about 80 or less, or about 60 or less, or about 50 or less.

In some preferred aspects, the silicon to aluminum ratio can be at least about 20, such as at least about 30 or at least about 40. In such embodiments, the silicon to aluminum ratio can optionally be about 80 or less, such as about 60 or less, or about 50 or less, or about 40 or less. Typically, reducing the silicon to aluminum ratio in a zeolite can result in a zeolite with a higher acidity, and therefore in higher activity for cracking of hydrocarbon or hydrocarbonaceous feeds, such as petroleum feeds. However, with respect to conversion of oxygenates to aromatics, such increased cracking activity may not be beneficial, and instead may result in increased formation of residual carbon or coke during the conversion reaction. Such residual carbon can deposit on the zeolite catalyst, leading to deactivation of the catalyst over time. Having a silicon to aluminum ratio of at least about 40, such as at least about 50 or at least about 60, can reduce and/or minimize the amount of additional residual carbon formed due to the acidic or cracking activity of the catalyst.

It is noted that the molar ratio described herein is a ratio of silicon to aluminum. If a corresponding ratio of silica to alumina were described, the corresponding ratio of silica (SiO₂) to alumina (Al₂O₃) would be twice as large, due to the presence of two aluminum atoms in each alumina stoichiometric unit. Thus, a silicon to aluminum ratio of 10 corresponds to a silica to alumina ratio of 20.

When used in the present catalyst composition, the zeolite can be present at least partly in the hydrogen (acid) form. Depending on the conditions used to synthesize the zeolite, this may correspond to converting the zeolite from, for example, the sodium form. This can readily be achieved, for example, by ion exchange to convert the zeolite to the ammonium form followed by calcination in air or an inert atmosphere at a temperature of about 400° C. to about 700° C. to convert the ammonium form to the active hydrogen form.

Additionally or alternately, the catalyst composition can include and/or be enhanced by a transition metal. Preferably the transition metal is a Group 12 metal from the IUPAC periodic table (sometimes designated as Group IIB) such as Zn and/or Cd. The transition metal can be incorporated into the zeolite by any convenient method, such as by impregnation or by ion exchange. After impregnation or ion exchange, the transition metal-enhanced catalyst can be treated in an oxidizing environment (air) or an inert atmosphere at a temperature of about 400° C. to about 700° C. The amount of transition metal can be related to the molar amount of aluminum present in the zeolite. Preferably, the molar amount of the transition metal can correspond to about 0.1 to about 1.3 times the molar amount of aluminum in the zeolite. For example, the molar amount of transition metal can be about 0.1 times the molar amount of aluminum in the zeolite, such as at least about 0.2 times, at least about 0.3 times, or at least about 0.4 times. Additionally or alternately, the molar amount of transition metal can be about 1.3 times or less relative to the molar amount of aluminum in the zeolite, such as about 1.2 times or less, about 1.0 times or less, or about 0.8 times or less. Still further additionally or alternately, the amount of transition metal can be expressed as a weight percentage of the self-bound or unbound zeolite, such as having at least about 0.1 wt % of transition metal, at least about 0.25 wt %, at least about 0.5 wt %, at least about 0.75 wt %, or at least about 1.0 wt %. Additionally or alternately, the amount of transition metal can be about 20 wt % or less, such as about 10 wt % or less, about 5 wt % or less, about 2.0 wt % or less, about 1.5 wt % or less, about 1.2 wt % or less, about 1.1 wt % or less, or about 1.0 wt % or less.

Additionally or alternately, the catalyst composition can be substantially free of phosphorous. A catalyst composition that is substantially free of phosphorous can contain about 0.01 wt % of phosphorous or less, such as less than about 0.005 wt % or less than about 0.001 wt % of phosphorous. A catalyst composition substantially free of phosphorous can be substantially free of intentionally added phosphorous or substantially free of both intentionally added phosphorous as well as phosphorous present as an impurity in a reagent for forming the catalyst composition. Additionally or alternately, the catalyst composition can contain no added phosphorous, such as containing no intentionally added phosphorous and/or containing no phosphorous impurities to within the detection limits of standard methods for characterizing a reagent and/or a resulting zeolite.

The catalyst composition can employ the transition metal-enhanced zeolite in its original crystalline form or after formulation into catalyst particles, such as by extrusion. A process for producing zeolite extrudates in the absence of a binder is disclosed in, for example, U.S. Pat. No. 4,582,815, the entire contents of which are incorporated herein by reference. Preferably, the transition metal can be incorporated after formulation of the zeolite (such as by extrusion) to form self-bound catalyst particles. Optionally, a self-bound catalyst can be steamed after extrusion.

The transition metal-enhanced zeolite catalyst composition employed herein can further be characterized by at least one, preferably at least two, and more preferably all of the following properties: (a) a mesoporosity (i.e., mesopore surface area or surface area external to the zeolite) of greater than about 20 m²/g, such as greater than about 30 m²/g; (b) a microporous surface area of at least about 340 m²/g, such as at least about 350 m²/g or at least about 370 m²/g; and (c) a diffusivity for 2,2-dimethylbutane of greater than about 1.0×10⁻² sec⁻¹, such as greater than about 1.25×10⁻² sec⁻¹, when measured at a temperature of about 120° C. and a 2,2-dimethylbutane pressure of about 60 torr (about 8 kPa).

Of these properties, mesoporosity and diffusivity for 2,2-dimethylbutane are determined by a number of factors for a given zeolite, including the crystal size of the zeolite. Microporous surface area is determined by the pore size of the zeolite and the availability of the zeolite pores at the surfaces of the catalyst particles. Producing a zeolite catalyst with the desired low (minimum) mesoporosity, microporous surface area, and 2,2-dimethylbutane diffusivity would be well within the expertise of anyone of ordinary skill in zeolite chemistry. It is noted that mesopore surface area and micropore surface area can be characterized, for example, using adsorption-desorption isotherm techniques within the expertise of one of skill in the art, such as the BET (Brunauer Emmet Teller) method.

It is noted that the micropore surface area can be characterized for zeolite crystals or a catalyst formed from the zeolite crystals. In various aspects, the micropore surface area of a self-bound catalyst or a catalyst formulated with a separate binder can be at least about 340 m²/g, such as at least about 350 m²/g, at least about 370 m²/g, or at least about 380 m²/g. Typically, a formulation of zeolite crystals into catalyst particles (either self-bound or with a separate binder) can result in some loss of micropore surface area relative to the micropore surface area of the zeolite crystals. Thus, in order to provide a catalyst having the desired micropore surface area, the zeolite crystals can also have a micropore surface area of at least about 340 m²/g, such as at least about 350 m²/g, at least about 360 m²/g, at least about 370 m²/g, or at least about 380 m²/g. As a practical matter, the micropore surface area of a zeolite crystal and/or a corresponding self-bound or bound catalyst as described herein can be less than about 1000 m²/g, and typically less than about 750 m²/g. Additionally or alternately, the micropore surface area of a catalyst (self-bound or with a separate binder) can be about 105% or less of the micropore surface area of the zeolite crystals in the catalyst, and typically about 100% or less of the micropore surface area of the zeolite crystals in the catalyst, such as from about 80% to about 100% of the micropore surface area of the zeolite crystals in the catalyst. For example, the micropore surface area of a catalyst can be at least about 80% of the micropore surface area of the zeolite crystals in the catalyst, such as at least about 85%, at least about 90%, at least about 95%, at least about 97%, or at least about 98%, and/or about 100% or less, about 99% or less, about 98% or less, about 97% or less, or about 95% or less.

Additionally or alternately, the diffusivity for 2,2-dimethylbutane of a catalyst (self-bound or with a separate binder) can be about 105% or less of the diffusivity for 2,2-dimethylbutane of the zeolite crystals in the catalyst, and typically about 100% or less of the diffusivity for 2,2-dimethylbutane of the zeolite crystals in the catalyst, such as from about 80% to about 100% of the diffusivity for 2,2-dimethylbutane of the zeolite crystals in the catalyst. For example, the diffusivity for 2,2-dimethylbutane of a catalyst can be at least about 80% of the diffusivity for 2,2-dimethylbutane of the zeolite crystals in the catalyst, such as at least about 85%, at least about 90%, at least about 95%, at least about 97%, or at least about 98%, and/or about 100% or less, about 99% or less, about 98% or less, about 97% or less, or about 95% or less.

In some aspects, the zeolite catalyst can have an alpha value of at least about 10, such as at least about 20 or at least about 50. Alpha value is a measure of the acid activity of a zeolite catalyst as compared with a standard silica-alumina catalyst. The alpha test is described in U.S. Pat. No. 3,354,078; in the Journal of Catalysis at vol. 4, p. 527 (1965), vol. 6, p. 278 (1966), and vol. 61, p. 395 (1980), each incorporated herein by reference as to that description. The experimental conditions of the test used herein include a constant temperature of about 538° C. and a variable flow rate as described in detail in the Journal of Catalysis at vol. 61, p. 395. The higher alpha values correspond with a more active cracking catalyst.

As an alternative to forming self-bound catalysts, zeolite crystals can be combined with a binder to form bound catalysts containing a relatively small amount of binder. Suitable binders for zeolite-based catalysts can include various inorganic oxides, such as silica, alumina, zirconia, titania, silica-alumina, cerium oxide, magnesium oxide, or combinations thereof. Generally, a binder can be present in an amount of about 5 wt % or less, such as about 1 wt % or less. Combining the zeolite and the binder can generally be achieved, for example, by mulling an aqueous mixture of the zeolite and binder and then extruding the mixture into catalyst pellets. A process for producing zeolite extrudates using a silica binder is disclosed in, for example, U.S. Pat. No. 4,582,815. Optionally, a bound catalyst can be steamed after extrusion.

In some aspects, a binder can be used that is substantially free of alumina, such as a binder that is essentially free of alumina. In this description, a binder that is substantially free of alumina is defined as a binder than contains about 10 wt % alumina or less, such as about 7 wt % or less, about 5 wt % or less, or about 3 wt % or less. A binder that is essentially free of alumina is defined as a binder that contains about 1 wt % or less, such as about 0.5 wt % or less or about 0.1 wt % or less. Additionally or alternately, a binder can be used that contains no intentionally added alumina and/or that contains no alumina within conventional detection limits for determining the composition of the binder and/or the reagents for forming the binder. Although alumina is commonly used as a binder for zeolite catalysts, due in part to ease of formulation of alumina-bound catalysts, in some aspects the presence of alumina in the binder can reduce and/or inhibit the activity of a transition metal-enhanced zeolite for converting methanol to aromatics. For example, for a catalyst where the transition metal is incorporated into the catalyst after formulation of the bound catalyst (such as by extrusion), the transition metal may have an affinity for exposed alumina surfaces relative to exposed zeolite surfaces, leading to increased initial deposition and/or migration of transition metal to regions of the bound catalyst with an alumina surface in favor of regions with a zeolite surface. Additionally or alternately, alumina-bound catalysts can tend to have low micropore surface area, meaning that the amount of available zeolite surface available for receiving a transition metal may be undesirably low.

In some aspects, a binder for formulating a catalyst can be selected so that the resulting bound catalyst has a micropore surface area of at least about 340 m²/g, such as at least about 350 m²/g or at least about 370 m²/g. An example of a suitable binder for forming bound catalysts with a desirable micropore surface area is a silica binder. Optionally but preferably, a suitable binder can be a binder with a surface area of about 200 m²/g or less, such as about 175 m²/g or less or about 150 m²/g or less. Without being bound by any particular theory, it is believed that catalysts formed using high surface area binders (such as high surface area alumina binders) can have an increased tendency for deposited transition metals to migrate to the binder, rather than remaining associated with the zeolite. Unless otherwise specified, the surface area of the binder is defined herein as the combined micropore surface area and mesopore surface area of the binder.

Feedstocks and Products

In various aspects, catalysts described herein can be used for conversion of oxygenate feeds to aromatics and/or olefins products, such as oxygenates containing at least one C1-C4 alkyl group (e.g., oxygenates containing at least one C1-C3 alkyl group). Examples of suitable oxygenates include feeds containing methanol, dimethyl ether, C1-C4 alcohols, ethers with C1-C4 alkyl chains, including both asymmetric ethers containing C1-C4 alkyl chains (such as methyl ethyl ether, propyl butyl ether, or methyl propyl ether) and symmetric ethers (such as diethyl ether, dipropyl ether, or dibutyl ether), or combinations thereof. It is noted that oxygenates containing at least one C1-C4 alkyl group are intended to explicitly identify oxygenates having alkyl groups containing about 4 carbons or less. Preferably the oxygenate feed can include at least about 50 wt % of one or more suitable oxygenates, such as at least about 75 wt %, at least about 90 wt %, or at least about 95 wt %. Additionally or alternately, the oxygenate feed can include at least about 50 wt % methanol, such as at least about 75 wt % methanol, at least about 90 wt % methanol, or at least about 95 wt % methanol. The oxygenate feed can be derived from any convenient source. For example, the oxygenate feed can be formed by reforming of hydrocarbons in a natural gas feed to form synthesis gas (H₂, CO, CO₂), and then using the synthesis gas to form alcohols.

In various aspects, the yield of aromatics relative to the total hydrocarbon product can be at least about 53 wt %, such as at least about 55 wt %, at least about 57 wt %, or at least about 59 wt %. In some aspects, the total C5+ (liquid) product yield can be greater than the yield of aromatics by about 10 wt % or less, such as greater than the yield of aromatics by about 8 wt % or less or by about 6 wt % or less. For example, in an aspect where the yield of aromatics relative to the total hydrocarbon product is about 60 wt %, the yield of total liquid product can be about 65 wt %. In some aspects, the combined yield of aromatics and olefins in the total hydrocarbon product can be at least about 70 wt %, such as at least about 75 wt % or at least about 80 wt %. It is noted that olefins in the hydrocarbon product can include (gaseous) C2-C4 olefins, and therefore the amount of aromatics plus olefins in the total hydrocarbon product may not necessarily directly relate to the C5+ (liquid) yield in the total hydrocarbon product.

In still other aspects, converting oxygenates to aromatics and/or olefins in the presence of a catalyst as described herein can be beneficial for producing a mixture of aromatics with a desirable composition. For example, in some aspects, converting oxygenates to aromatics as described herein can result in production of aromatics with an increased percentage of C9 and C10 aromatics relative to the total aromatics produced. In additional or alternate aspects, production of C6 and C7 aromatics can be enhanced.

In yet other aspects, converting an oxygenate feed to aromatics in the presence of a catalyst as described herein can reduce and/or minimize the amount of coke formation that occurs during conversion.

Suitable and/or effective conditions for performing a conversion reaction can include temperatures between about 150° C. to about 550° C., total pressures between about 0.1 psia (about 0.7 kPaa) to about 500 psia (about 3.5 MPaa), and an oxygenate space velocity between about 0.1 h⁻¹ to about 20 h⁻¹, based on weight of oxygenate relative to weight of catalyst. For example, the temperature can be at least about 375° C., such as at least about 400° C., at least about 450° C., or at least about 460° C. Additionally or alternately, the temperature can be about 550° C. or less, such as about 525° C. or less or about 500° C. or less.

It is noted that the oxygenate feed and/or conversion reaction environment can include water in various proportions. Conversion of oxygenates to aromatics and olefins results in production of water as a product, so the relative amounts of oxygenate (such as methanol or dimethyl ether) and water can vary within the reaction environment. Based on the temperatures present during methanol conversion, the water in the reaction environment can result in “steaming” of a catalyst. Thus, a catalyst used for conversion of oxygenates to aromatics is preferably a catalyst that substantially retains activity when steamed. Water may also be present in a feed prior to contacting the zeolite catalyst. For example, in commercial processing of methanol to form gasoline, in order to control heat release within a reactor, an initial catalyst stage may be used to convert a portion of the methanol in a feed to dimethyl ether and water prior to contacting a zeolite catalyst for forming gasoline.

ADDITIONAL EMBODIMENTS Embodiment 1

A catalyst composition comprising a self-bound zeolite and a Group 12 transition metal selected from the group consisting of Zn, Cd, or a combination thereof, the zeolite having a silicon to aluminum ratio of at least about 10, such as at least about 20, or at least about 30, or at least about 40, the catalyst composition having a micropore surface area of at least about 340 m²/g, a molar ratio of Group 12 transition metal to aluminum of about 0.1 to about 1.3, and at least one of: (a) a mesoporosity of greater than about 20 m²/g; and (b) a diffusivity for 2,2-dimethylbutane of greater than about 1×10⁻² sec⁻¹ when measured at a temperature of about 120° C. and a 2,2-dimethylbutane pressure of about 60 torr (about 8 kPa).

Embodiment 2

The catalyst composition of Embodiment 1, wherein the catalyst composition has an alpha value of at least about 10, such as at least about 20 or at least about 50.

Embodiment 3

The catalyst composition of any of the above embodiments, wherein the catalyst composition has a mesoporosity of greater than about 30 m²/g.

Embodiment 4

The catalyst composition of any of the above embodiments, wherein the catalyst composition has a microporous surface area of at least about 350 m²/g, such as at least about 370 m²/g or at least about 380 m²/g.

Embodiment 5

The catalyst composition of any of the above embodiments, wherein the catalyst composition has a diffusivity for 2,2-dimethylbutane of greater than about 1.25×10⁻² sec⁻¹ when measured at a temperature of about 120° C. and a 2,2-dimethylbutane pressure of about 60 torr (8 kPa), such as greater than about 1.5×10⁻² sec⁻¹.

Embodiment 6

The catalyst composition of any of the above embodiments, wherein the zeolite has a constraint index of about 1 to about 12.

Embodiment 7

The catalyst composition of any of the above embodiments, wherein the zeolite comprises ZSM-5, ZSM-11, a zeolite having an MFI framework, a zeolite having an MEL framework, or a combination thereof.

Embodiment 8

The catalyst composition of any of the above embodiments, wherein the zeolite is ZSM-5.

Embodiment 9

The catalyst composition of any of the above embodiments, wherein the zeolite is ZSM-11.

Embodiment 10

The catalyst composition of any of the above embodiments, wherein the silicon to aluminum molar ratio of the zeolite is from about 20 to about 100, such as at least about 30 or at least about 40, and/or about 80 or less, about 60 or less, or about 50 or less.

Embodiment 11

The catalyst composition of any of the above embodiments, wherein the amount of Group 12 transition metal is from about 0.1 wt % to about 2 wt % of the total catalyst composition.

Embodiment 12

The catalyst composition of any of the above embodiments, wherein the catalyst composition has both a mesoporosity of greater than about 20 m²/g and a diffusivity for 2,2-dimethylbutane of greater than about 1×10⁻² sec⁻¹ when measured at a temperature of about 120° C. and a 2,2-dimethylbutane pressure of about 60 torr (about 8 kPa).

Embodiment 13

A process for organic compound conversion employing the catalyst composition of any of Embodiments 1-12.

Embodiment 14

The process of Embodiment 13, wherein said organic compound conversion comprises the conversion of oxygenates containing at least one C1-C4 alkyl group to aromatics, the oxygenates containing at least one C1-C4 alkyl group preferably comprising at least one of methanol, dimethyl ether, symmetric ethers containing C1-C4 alkyl groups, asymmetric ethers containing C1-C4 alkyl groups, C1-C4 alcohols, or a combination thereof.

EXAMPLES

The following examples show data from testing and analysis of a variety of ZSM-5 self-bound catalysts that were used for performing a methanol to aromatics reaction. After forming ZSM-5 with a desired micropore surface area, the catalysts were formed into self-bound catalyst particles using a procedure similar to the following. It is noted that the absolute values in grams provided below should be considered as representative of using an appropriate ratio of the various components.

ZSM-5 crystal (about 1,400 grams on a solids basis) was added to a mixer and dry mulled. Then, approximately 190 grams of deionized water was added during mulling. After about 10 minutes, about 28 grams of about 50 wt % caustic solution mixed with about 450 grams of deionized water were added to the mixture and mulled for an additional 5 minutes. The mixture was then extruded into ˜ 1/10″ quadralobes. The extrudates were dried overnight at about 250° F. (about 121° C.) and then calcined in nitrogen for about 3 hours at about 1000° F. (about 538° C.). The extrudates were then exchanged twice with a 1N solution of ammonium nitrate. The exchanged crystal was dried overnight at about 250° F. (about 121° C.) and then calcined in air for about 3 hours at about 1000° F. (about 538° C.).

In the following examples, some of the ZSM-5 catalyst includes a transition metal, such as Zn or Cd. To form the transition metal-enhanced catalysts described below, a self-bound catalyst as described above was impregnated via incipient wetness with a solution containing the desired metal for impregnation. The impregnated crystal was then dried overnight at about 250° F. (about 121° C.) and then calcined in air for about 3 hours at about 1000° F. (about 538° C.).

In the discussion below, references to the micropore surface area of a catalyst correspond to the micropore surface area of the self-bound catalyst. The procedure for self-binding may cause some reduction in micropore surface area of the catalyst relative to the micropore surface area of the corresponding zeolite crystals prior to self-binding.

Example 1 Micropore Surface Area Versus Aromatics Yield

In various aspects, suitable methanol conversion catalysts can have sufficient microporous surface area, such as a micropore surface area of at least about 340 m²/g or at least about 350 m²/g. Micropore surface areas of six example ZSM-5 catalysts, all with about 0.85-1 wt % Zn, are shown in Table 1. The aromatic content (wt % of hydrocarbon product) during methanol conversion at about 450° C., an inlet feed pressure of about 15 psig (about 110 kPag) and about 20 WHSV (g-CH₃OH g-catalyst⁻¹ h⁻¹) is also shown in Table 1. The inlet feed was a mixture of methanol and water, to provide about 13.5 psig (about 95 kPag) of CH₃OH and about 1.5 psig (about 11 kPag) of H₂O. The reaction conditions used for methanol conversion in Table 1 appeared to result in substantially complete conversion of the methanol in the feed. Unless otherwise indicated, the methanol conversion reactions were performed in a tubular stainless steel reactor. Note that in Table 1 (as well as tables in the other Examples), the ZSM-5 is described in terms of a ratio of Si to Al₂. The corresponding silicon to aluminum ratio is half of the value of the Si to Al₂ ratio.

For the catalysts in Table 1, all of the catalysts correspond to self-bound catalysts. As shown in Table 1, the resulting aromatics weight percent in the total hydrocarbon product appeared strongly correlated with the micropore surface area, although other factors can also influence the final amount of aromatics in the hydrocarbon product.

TABLE 1 Micropore Surface Area versus Aromatics Yield Micropore Aromatics (wt %) Surface in hydrocarbon Catalyst Si/Al₂ Area (m²/g) product Zn/H-ZSM-5 ~60 ~193 ~31 Zn/H-ZSM-5 ~60 ~218 ~29 Zn/H-ZSM-5 ~60 ~277 ~34 Zn/H-ZSM-5 ~60 ~375 ~54 Zn/H-ZSM-5 ~60 ~376 ~57 Zn/H-ZSM-5 ~60 ~383 ~54

Example 2 Micropore Surface Area Versus Aromatics Yield

Example 1 demonstrated the impact of micropore surface area on aromatics production for various Zn/H-ZSM-5 catalysts. In this example, the impact of including Zn as a transition metal was demonstrated. Table 2 shows the methanol to aromatics selectivity for ZSM-5 catalysts of different micropore surface area, with and without added Zn. As shown in Table 2, aromatic selectivity appeared to increase only modestly with micropore surface area for ZSM-5 catalysts that do not include an additional transition metal. For a low micropore surface area ZSM-5 catalyst, addition of a transition metal does not appear to provide a clear benefit in aromatics selectivity. However, for a catalyst according to the invention including both a high micropore surface area and a transition metal (Zn in this example), a substantial increase in aromatics selectivity appeared to be achieved. It is noted that the maximum aromatic yields for H-ZSM-5 and Zn/H-ZSM-5 catalysts were achieved on a small ZSM-5 crystal.

For the data in Table 2, the effects of micropore surface area on aromatic yields from methanol conversion are shown for catalysts corresponding to self-bound H-ZSM-5 and Zn-modified H-ZSM-5 catalysts. The methanol conversion reaction used to generate the results in Table 2 was performed at about 500° C., about 13.5 psig (about 95 kPag) CH₃OH, about 1.5 psig (about 11 kPag) H₂O (inlet pressures), and about 20 WHSV (g-CH₃OH g-catalyst⁻¹ h⁻¹). These conditions appeared to result in substantially complete conversion of the methanol in the feed.

TABLE 2 Micropore Surface Area and Metals Content versus Aromatics Yield Micropore Zn Aromatics (wt %) Surface content in hydrocarbon Catalyst Si/Al₂ Area (m²/g) (wt %) product H-ZSM-5 ~60 >350 — ~29 H-ZSM-5 ~60 <340 — ~27 H-ZSM-5 ~60 <250 — ~25 Zn/H-ZSM-5 ~60 >350 ~0.93 ~57 Zn/H-ZSM-5 ~60 <250 ~0.86 ~22

Example 3 Combined Aromatics and Olefins Yield and Residual Carbon

Both aromatics and olefins are typically considered higher value products than paraffins in methanol conversion. Table 3 shows the combined aromatics and olefin (A+O) selectivity as well as the residual carbon deposited on the catalyst (measured post-reaction, ex-situ via a thermogravimetric method) for various H-ZSM-5 and Zn/H-ZSM-5 catalysts. In Table 3, the wt % of coke or residual carbon is expressed as a wt % relative to the weight of catalyst. The residual carbon present on the ZSM-5 catalysts after performing methanol conversion can be an indicator of how rapidly such catalysts will deactivate (via coking) during extended periods of methanol conversion.

For catalysts without an additional transition metal, the A+O selectivity appeared similar for catalysts with micropore surface areas greater than about 250 m²/g at a given Si/Al₂ ratio. However, coking appeared to be exacerbated for the H-ZSM-5 catalysts with micropore surface area less than about 340 m²/g. The amount of coking also appeared to increase for H-ZSM-5 catalysts with a ratio of Si to Al_(e) of about 30 or less. For catalysts including a transition metal, but with a micropore surface area less than the about 340 m²/g for a catalyst according to the invention, increased coking may be related to a low availability of reaction sites for forming aromatics relative to acidic reaction sites that can cause, for example, cracking. Similarly, for catalysts including a transition metal but with a low diffusivity, the low diffusivity can indicate an increased diffusion time (potentially equivalent to an increased diffusion length) for compounds trying to exit a catalyst pore. An increase in diffusion length and/or diffusion time for compounds in the zeolite catalysts could lead to further unsaturation of hydrogen-deficient species (and coking) before such species can escape the zeolite pores.

The presence of Zn on H-ZSM-5 catalysts with a micropore surface area greater than about 350 m²/g appeared to increase the A+O selectivity to about 81%, compared to an about 49% A+O selectivity for H-ZSM-5 without a transition metal. It is noted that the A+O selectivity for a catalyst with Zn on H-ZSM-5 with a lower micropore surface area appeared to be about 71%. Although the combined A+O yield seemed high, as shown previously in Example 2, the aromatics portion of the combined A+O yield was only about 30 wt %, meaning that the majority of the A+O selectivity was likely due to production of olefins. While olefins are generally a desirable product relative to paraffins, aromatics are typically a still more desirable product.

Table 3 shows aromatic plus olefin (A+O) selectivity and residual carbon remaining on catalyst after reaction of a methanol feed in the presence of H-ZSM-5 and Zn-modified H-ZSM-5 catalysts. The reaction conditions for generating the results in Table 3 included a temperature of about 500° C., a methanol partial pressure (inlet) of about 13.5 psig (about 95 kPag) CH₃OH, a water partial pressure (inlet) of about 1.5 psig (about 11 kPag) H₂O, and a space velocity of about 20 WHSV (g-CH₃OH g-catalyst⁻¹ h⁻¹). The reaction conditions appeared to be sufficient to cause substantially complete conversion of the methanol in the feed.

TABLE 3 Aromatics Plus Olefin Yield and Residual Carbon Aromatics Residual Micropore plus carbon Surface olefins (wt %) 2,2-DMB Area (wt %) in after diffusivity Catalyst Si/Al₂ (m²/g) product reaction (sec⁻¹) H-ZSM-5 ~60 >350 ~49 ~0.5 ~4.9 × 10⁻² H-ZSM-5 ~30 >350 ~45 ~5.1 ~1.7 × 10⁻² H-ZSM-5 ~60 <340 ~49 ~1.2 <N/A> H-ZSM-5 ~30 <340 ~45 ~6.1 ~3.1 × 10⁻³ Zn/H-ZSM-5 ~60 >350 ~81 ~0.1 ~1.5 × 10⁻¹ Zn/H-ZSM-5 ~60 <250 ~71 ~6.0 ~7.7 × 10⁻²

As shown in Table 3, various factors can result in increased coke formation on a catalyst. In order to avoid coke formation, a combination of a moderate silicon to aluminum ratio, a sufficiently high micropore surface area, and a sufficiently high 2,2-DMB diffusivity can lead to reduced coke formation on a catalyst. Note that diffusivity data was not obtained for the process run corresponding to row 3 in Table 3.

Example 4 Comparison of Group 12 (Group IIB) Transition Metals

Table 4 demonstrates the advantage of Zn or Cd addition to self-bound ZSM-5 catalysts, as compared to equivalent self-bound ZSM-5 without modification by inclusion of a transition metal. The unmodified and transition metal-modified catalysts were tested for methanol conversion at a temperature of about 500° C., a total inlet pressure of about 15 psig (about 110 kPag) (about 13.5 psig/95 kPag, CH₃OH, about 1.5 psig/11 kPag H₂O), and a space velocity of about 20 WHSV (g-CH₃OH g-catalyst⁻¹ h⁻¹). The reaction conditions appeared to result in substantially complete conversion of the methanol in the feed.

TABLE 4 Comparison of Group 12 Transition Metals Micropore Surface Metal Aromatics Area Metal content (wt %) in Example Si/Al₂ (m²/g) promoter (wt %) product 1 ~60 >350 — — ~30 2 ~60 >350 Zn ~0.93 ~57 3 ~60 >350 Cd ~0.75 ~54

Example 5 Distribution of Aromatic Products

The methanol conversion results shown in Table 4 were further characterized to determine the distribution of types of aromatic compounds in the aromatics yield. Table 5 shows the distribution of C6-C10 aromatic species as a weight percent of the total aromatics product for each of the methanol conversion runs shown in Table 4.

TABLE 5 Distribution of Aromatic Products C6 C7 C8 C9 C10+ % of % of % of % of % of Catalyst aromatics aromatics aromatics aromatics aromatics ZSM-5 ~7 ~26 ~42 ~19 ~6 Zn/H-ZSM-5 ~5 ~19 ~37 ~22 ~19 Cd/H-ZSM-5 ~10 ~32 ~38 ~15 ~5

As shown in Table 5, the transition metal included on the catalyst appeared to impact the distribution of aromatic products formed during methanol conversion. Use of Zn as the additional transition metal appeared to result in production of a larger percentage the heavier C9 and C10+ aromatic compounds at the expense of production of C6 and C7 compounds. By contrast, use of Cd as the additional transition metal appeared to result in production of additional C6 and C7 compounds.

Example 6 Aromatics Yield Relative to Liquid Product Yield

The methanol to conversion examples shown in Tables 1-5 correspond to reaction conditions with high space velocities of methanol feed relative to the amount of catalyst. In a commercial setting, lower space velocities are likely to be preferred, such as space velocities (WHSV) between about 0.1 and about 5.0 g-CH₃OH g-catalyst⁻¹ h⁻¹. Table 6 shows an example of hydrocarbon product liquid yields as well as the aromatic fraction of such yields for an H-ZSM-5 catalyst and a corresponding Zn/H-ZSM-5 catalyst at a WHSV of about 2, representing more typical commercial operating conditions for a reactor. The liquid yield corresponds to the yield of C5+ compounds. The results in Table 6 were generated at a temperature of about 450° C. and a total pressure of about 15 psig (about 110 kPag). It is noted that for this example, the feed was substantially composed of methanol (trace amounts of water may have been present). The catalysts correspond to catalysts with a micropore surface area of greater than about 350 m²/g.

TABLE 6 Aromatics Yield versus Liquid Yield liquid yield/ Aromatics yield/ wt % of wt % of MeOH hydrocarbon hydrocarbon Catalyst conversion/% product product H-ZSM-5 ~99.8 ~48.6 ~40.4 Zn/H-ZSM-5 ~99.8 ~64.9 ~60.3

As shown in Table 6, at a WHSV of less than about 5, the aromatics yield for both the catalysts with and without additional transition metal appeared different from the liquid yield by about 10 wt % or less, indicating that a relatively low weight percentage of paraffins were being generated. As shown in Table 6, addition of the additional transition metal not only appeared to improve the aromatics yield to greater than about 60 wt %, but also the difference between the aromatics yield and the liquid yield appeared to be less than 5 wt %. Thus, the additional transition metal not only appeared to improve the aromatics yield, but also apparently reduced the yield of the less desirable C5+ paraffin type compounds.

Example 7 Regeneration of Catalyst

ZSM-5 catalysts can suffer both reversible and irreversible deactivation via coking, and steaming, respectively, during methanol conversion. The increased selectivity to aromatic products on Zn/H-ZSM-5 catalysts could promote the formation of polynuclear arene species, which are known precursors to coke. Regeneration of Zn/H-ZSM-5 catalysts was performed by treating the spent catalyst in air to about 850° F. (about 454° C.). Table 7 shows the Zn content and the aromatic selectivity of fresh and regenerated Zn/H-ZSM-5 samples. The regenerated Zn/H-ZSM-5 sample appeared to regain ˜90+% of the selectivity to aromatics, compared to the fresh sample. The catalysts were tested for methanol conversion at a temperature of about 500° C., a total pressure of about 15 psig (about 110 kPag) (about 13.5 psig/95 kPag, CH₃OH, about 1.5 psig/11 kPag H₂O), and a space velocity of about 20 WHSV (g-CH₃OH g-catalyst⁻¹ h⁻¹). The reaction conditions appeared to result in substantially complete conversion of the methanol in the feed.

TABLE 7 Catalyst Activity after Regeneration Micropore Surface Zn Aromatics Area content (wt %) Catalyst Si/Al₂ (m²/g) (wt %) in product Zn/H-ZSM-5 ~60 >350 ~0.93 ~57 (fresh) Zn/H-ZSM-5 ~60 >350 ~0.93 ~52 (regen)

While the present invention has been described and illustrated by reference to particular embodiments, those of ordinary skill in the art will appreciate that the invention lends itself to variations not necessarily illustrated herein. For this reason, then, reference should be made solely to the appended claims for purposes of determining the true scope of the present invention. 

What is claimed is:
 1. A catalyst composition comprising a self-bound zeolite having an MFI or MEL framework and a Group 12 transition metal selected from the group consisting of Zn, Cd, or a combination thereof, the zeolite having a silicon to aluminum ratio of at least about 10, the catalyst composition having a micropore surface area of at least about 340 m²/g, a molar ratio of Group 12 transition metal to aluminum of about 0.1 to about 1.3, and at least one of: (a) a mesopore surface area of greater than about 20 m²/g; and (b) a diffusivity for 2,2-dimethylbutane of greater than 1×10⁻² sec⁻¹ when measured at a temperature of about 120° C. and a 2,2-dimethylbutane pressure of about 60 torr (about 8 kPa).
 2. The catalyst composition of claim 1, wherein the catalyst composition has an alpha value of at least about
 10. 3. The catalyst composition of claim 1, wherein the catalyst composition has a mesopore surface area of greater than about 30 m²/g.
 4. The catalyst composition of claim 1, wherein the catalyst composition has a microporous surface area of at least about 350 m²/g.
 5. The catalyst composition of claim 1, wherein the catalyst composition has a diffusivity for 2,2-dimethylbutane of greater than about 1.25×10⁻¹ sec⁻¹ when measured at a temperature of about 120° C. and a 2,2-dimethylbutane pressure of about 60 torr (about 8 kPa).
 6. The catalyst composition of claim 1, wherein the zeolite has a constraint index of about 1 to about
 12. 7. The catalyst composition of claim 1, wherein the zeolite comprises ZSM-5, ZSM-11 or a combination thereof.
 8. The catalyst composition of claim 1, wherein the zeolite is ZSM-5.
 9. The catalyst composition of claim 1, wherein the zeolite is ZSM-11.
 10. The catalyst composition of claim 1, wherein the silicon to aluminum molar ratio of the zeolite is from about 20 to about
 100. 11. The catalyst composition of claim 1, wherein the amount of Group 12 transition metal is about 0.1 wt % to about 2 wt % of the total catalyst composition.
 12. The catalyst composition of claim 1, wherein the catalyst composition has both a mesopore surface area of greater than about 20 m²/g and a diffusivity for 2,2-dimethylbutane of greater than about 1×10⁻² sec⁻¹ when measured at a temperature of about 120° C. and a 2,2-dimethylbutane pressure of about 60 torr (about 8 kPa).
 13. A catalyst composition comprising a self-bound zeolite having an MFI or MEL framework and a Group 12 transition metal selected from the group consisting of Zn, Cd, or a combination thereof, the zeolite having a silicon to aluminum ratio of at least about 20, and the catalyst composition having a micropore surface area of at least about 340 m²/g, a molar ratio of Group 12 transition metal to aluminum of about 0.1 to about 1.3, a mesopore surface area of greater than about 20 m²/g, and a diffusivity for 2,2-dimethylbutane of greater than 1×10⁻² sec⁻¹ when measured at a temperature of about 120° C. and a 2,2-dimethylbutane pressure of about 60 torr (about 8 kPa). 